The present invention relates generally to the fields of x-ray imaging and digital x-ray detectors. More specifically, the present invention relates to a solid state x-ray detector which may rapidly stabilize under dynamic mechanical loads.
Prior art digital x-ray detectors may be generally constructed in the following manner. Starting with a substrate with an interior and exterior surface, a number of detector elements are arranged onto the exterior surface of the substrate. The arrangement creates an array of detector elements. Each detector element includes a scintillator and a photosensor. The scintillator converts x-ray energy into light energy. The photosensor, in turn, is sensitive to the visible light energy. A layer of absorptive material, such as black or dark colored vinyl, is located on the interior surface of the substrate. The absorptive material absorbs light and heat emitted from the detectors during x-ray detection. Supporting the material, a base or a frame that is grounded may be provided. The base may be grounded to a chassis, earth, or to any other suitable common.
The substrate may comprise an insulating material such as glass. Alternatively, the substrate may comprise a conductive material. In the case of the substrate comprising a conductive material, a non-conductive material may be applied to the exterior surface before arranging the array of detector elements.
Present day solid state photosensors for use in x-ray imaging are typically formed from amorphous silicon photodiodes. Amorphous silicon photodiodes have an intrinsic capacitance. Thus, each photodiode acts like a capacitor—e.g. the photodiode may be charged to a charged voltage, or it may be discharged.
In order to obtain an accurate x-ray image using a digital detector, it is necessary to compensate for variations between individual detector elements. Compensation may be accomplished by taking at least two separate readings from individual detector elements to generate a single x-ray image, for example.
The following method may be used to generate an x-ray image with solid state detectors. First, each detector element is charged to a charged voltage, based on each photodiode's intrinsic capacitance. Then, an x-ray source provides x-rays to the detectors for a period of time T. Exposure to x-rays causes charge to be depleted from a detector element, and therefore for the voltage to drop across each photodiode. After exposure to x-rays, each detector element is recharged. During recharging, an amount of charge (or, alternately, current) that flows into each detector element is measured. Each recharge measurement represents an amount of x-ray energy detected by each detector element plus offset characteristics of each detector element. In other words, each recharge measurement represents a noisy signal at each detector element.
To compensate for the effects of noise/offset, a second measurement may be taken—a “dark image.” After an initial noisy signal measurement is made, a delay may occur, and then each detector element is recharged again. During recharging, an amount of current/charge that flows into each detector element is again measured. Each recharge measurement represents each detector element offset. The sum of these measurements for an array of detector elements is a “dark image,” because the dark image measurement is acquired without exposing the detector elements to x-rays. The dark image (second measurement) is subtracted from the noisy signal image (first measurement). In this manner, noise due to detector element variations may be accounted for in the final x-ray image, for example.
It may be preferable to delay before acquiring the dark image to account for system leakages. This delay period may be the same as the delay T between the initial charging of the detector array and the noisy signal image acquisition. Using this method, the steps for image acquisition may be as follows: charge the detector array; delay for a period of time T while exposing the array to x-rays; recharge array while measuring charge flow to obtain noisy signal image; delay for a period of time T; recharge the detector array while measuring charge flow to obtain dark image. Assuming the system leaks in a substantially repeatable manner over a period of time T, the image acquisition system may account for system leakages. Alternatively, the second delay period may be chosen to be a period of time other than T, based on known or assumed system charge leakage over time.
Measurements described above are sensitive, and involve relatively small amounts of charge or current. A variety of noise factors may reduce the accuracy of this measurement. One such noise factor is electromagnetic interference (EMI) due to static charge.
Digital x-ray detectors offer many advantages over conventional radiographic film cassette imaging systems. Radiographic film must be developed, which costs time and money. Film must be stored in a physical space. Also, film must be physically changed to make additional images.
In spite of these and other shortcomings, radiographic film detectors still have at least one important advantage over prior art digital x-ray detectors. Film-based detectors may accommodate dynamic mechanical loads without a significant loss in image quality. In contrast, prior art digital x-ray detectors take a relatively long time to stabilize after a shift in mechanical load. During stabilization, digital imaging systems produce degraded images. Every time the forces change, the system must stabilize again to provide accurate images.
One cause of digital x-ray detector destabilization is static electricity. As mechanical loads on a detector vary, the bottom surface of a detector glass plate rubs against other materials in the detector, such as an absorptive layer. Friction and contact cause static electricity to accumulate on the interior surface of the glass, which is non-dissipative. The absorptive layer, for instance, is a poor insulator, and prevents static charge from rapidly discharging into a conducting base of the detector, such as a metal base. As the charge slowly discharges from the glass plate to the base, system voltages and electric fields change. The solid state detector elements and measurement systems may be sensitive to these changing voltages and fields. The slow dissipation of static charge may be a significant source for error and noise in the measurement process. Slow dissipation of static charge may reduce the accuracy of both x-ray noisy signal measurements and dark image measurements.
Thus, there is a need for a digital x-ray detector system which may rapidly stabilize under dynamic mechanical loads. Additionally, there is a need for a digital x-ray detector system which may dissipate static charge to improve image quality.